1. Field of the Invention
The present invention relates to a method of controlling the amount of water which is added into a pulp washing system by monitoring the amount of water in a washed pulp mat leaving the pulp washing system.
2. Description of the Prior Art
Pollution control depends upon the washing operation control whereby if there is insufficient wash water added to the system the washing step is inefficient and increases the pollution.
Poor control can also be in the other direction whereby an excessive amount of wash water is applied and this excess water must be evaporated in the evaporator operation which then causes an excessive energy consumption.
Washing control systems presently in use do not use the water efficiently in that for short periods of time there is an excess of water used followed by a period whereby an insufficient amount is used. This leads to insufficiencies of both kinds described above within the one continuous washing system.
Pulp washing systems are designed to minimize the amount of fresh water needed to wash the black liquor from the pulp produced from a pulp digester. Countercurrent pulp washing techniques are almost exclusively used in order to increase efficiency in the pulp washing system. In conventional countercurrent washing operations, fresh water which is added to the system is generally referred to as shower water because it is sprayed as a shower upon a pulp mat which has been formed in the last of a number of washing operations.
Two important factors which must be understood with relation to the present washing control system are the dilution factor and the displacement factor. These factors influence the determination of the flow rate of shower water as calculated by the present control system and must necessarily be appreciated to enable the control system to permit a satisfactory pulp washing operation.
The dilution factor represents a ratio of the amount of fresh water sprayed onto a pulp mat undergoing a washing operation to the final volume of water contained in the pulp mat as it leaves the washing operation. The amount of fresh water entering the washing system may be expressed in appropriate flow rate units such as liters per minute. The volume of water leaving the washing system in the final washed pulp mat product is expressed in the same units. In countercurrent washing operations recycled water is sprayed on the pulp mat in all but the final washing operation. The dilution factor for each washing operation step prior to the final fresh water treatment is expressed as the ratio of the recycled water sprayed onto the pulp mat to the volume of water contained in the pulp mat which has been treated in the individual washing step.
The displacement ratio is equal to the fractional of the liquor entering the filter drum in the pulp mat which is displaced by water from the spray washer. The ideal displacement ratio would be 1.0 where ideal plug flow existed; however, the ideal situation is not obtained. The displacement factor is primarily a function of the dilution factor but is influenced by such factors as air entrainment in the pulp web, pulp web sheet uniformity, and the temperature of the system. The displacement factor can be determined by an analysis of the amount of dissolved solids which remain in the pulp after washing compared to the dissolved solids which would be in the pulp at the same consistency without any shower flow. In effect, this displacement ratio may be displayed by a comparison between the dissolved solids concentration in the water in the pulp mat exiting the washing system after the washing treatment, and the dissolved solids concentration of the pulp before the final shower water treatment.
While the use of countercurrent washing systems reduces the amount of fresh water which is needed in a pulp washing system, previous attempts to minimize the total amount of fresh shower water introduced into the wash system have proved to be inefficient. Previous systems have not provided for a continuous monitorization and immediate shower flow response to produce a pulp washing system which is continuously efficient in minimizing the shower flow necessary to produce a satisfactorily washed pulp product. The present invention overcomes the deficiencies of the control methods used in the past.
Primarily, two control methods have been used to control pulp washing systems in the past. In the first method a pulp flow rate on the entire set of washers is estimated to be constant and the pulp flow rate is calculated for one washer by correlating a flow measurement and a consistency. The consistency of the pulp leaving the washer drums is not taken into account except in the design of the system. The shower water flow on the last washer is then set by the operator based on hourly tests of the solids content of the liquor in the early stages of the washing operation. The system can be out of balance in both of the ways previously described several times during the hour without detection by the operator. The average liquor solids content can be on target yet the system can be inefficient in producing both high losses to the sewer and excessive water to be evaporated. This can be explained by showing that an overwash for part of the time cannot make up for an insufficient wash the other part of the time.
In the second prior art control method as described in U.S. Pat. No. 4,046,621 to Sexton, the conductivity of the liquid displaced from the pulp mat in the last washing step is measured and this measurement is used to adjust the amount of fresh washing liquid in the last washing stage.
This system is an improvement over the operator control alone but has several disadvantages. The first disadvantage is that conductivity is not precisely related to the liquor solids content as it is greatly influenced by the composition of the solids. Secondly, the large volumes of liquor circulated in the wash system have a large buffering action on the rate of change of liquor conductivity with a change in washing efficiency. In a typical pulp washing system operation at 500 metric tons of pulp per day the liquor volume maintained in each stage filtrate tank will be in the order of 200,000 liters which is recirculated in the wash system at a rate of about 30,000 liters per minute. The shower flow for 1.15 dilution factor would be 2928 liters per minute at 12 percent discharge consistency. Of this 2928 liters per minute approximately 382 liters per minute would penetrate the mat with a perfect displacement system. In a normal balanced system this 382 liters per minute would be mixing continuously with the 200,000 liters in the filtrate tank.
If the shower flow was accidently cut completely off the conductivity system of control would detect a rate of change of only (100.times.382/200,000)=0.19 percent per minute. This small change in conductivity would not initiate a change in the shower set point until significant inefficiencies in the system had occurred.